This is the 6201st most frequent German word.
deren
"Deren" is a German relative pronoun meaning "whose" (feminine or plural).
In this sentence, 'deren' is used as a relative pronoun in the genitive case referring to 'die Frau' and her children.
Das ist die Frau, deren Kinder ich unterrichte.
This is the woman whose children I teach.
Here, 'deren' is also a relative pronoun in the genitive case, referring to 'die Stadt' and its history.
Die Stadt, deren Geschichte mich interessiert, ist sehr alt.
The city whose history interests me is very old.
Again, 'deren' functions as a genitive relative pronoun referring to 'das Mädchen' and her book.